1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor light-receiving elements and more particularly to a method for efficiently and yieldingly manufacturing a PIN construction in a compound semiconductor thin film by vapor growth method grown on an InP substrate suitable for manufacturing the semiconductor light-receiving elements preferably applied to an optical communication system or measurement in a wavelength of greater than 0.9 .mu.m (general 11y, 1.25 to 1.60 .mu.m).
2. Description of the Related Arts
In manufacturing the semiconductor light-receiving elements, first, mixed crystal of InGaAs is grown on an InP substrate by vapor growth method to form a light-receiving layer and then a p-type impurity is introduced into the N-type light-receiving layer having a low carrier density to form a depletion portion. In the depletion portion, an optical signal which has been incident on the semiconductor light-receiving elements is converted into an electric signal. Normally, the depletion portion is formed by introducing a p-type impurity such as Zn, Cd, Be or Mg into the epitaxial crystal of the light-receiving layer by thermal diffusion method. Then, a film for preventing incident light from being reflected and a signal fetch electrode are formed on the surface of the semiconductor light-receiving elements.
The size of an epitaxial wafer is approximately one inch square due to the limiting condition of wafer processing. Therefore, the configuration of a wafer is not affected by the composition distribution of the epitaxial layer. In this situation, in order to manufacture a good quality semiconductor light-receiving elements, it is necessary that the composition distribution of the epitaxial layer is uniform, i.e., the composition of the epitaxial layer matches the crystal lattice of the substrate.
Suppose that the epitaxial layer has been formed on a substrate in a comparatively large size, for example, approximately two inches in diameter by vapor growth method in order to improve the production rate per wafer. As a result, the configuration of the wafer is affected by the composition distribution of the epitaxial layer and consequently convexes and concaves are formed on the wafer. The convexes and concaves thus formed cause concentration, which causes the formation of a defective crystal and increases dark current.
In the method for manufacturing the semiconductor light-receiving elements, temperatures for growing the epitaxial layer, for thermally diffusing a p-type impurity, and for forming an insulating film are higher than the room temperature. As a result, various thermal stresses act on the crystal of the epitaxial layer, which causes the formation of a defective crystal or the destruction of the crystal.
In forming a light-receiving layer consisting of InGaAs on a substrate consisting of InP, if the composition of the epitaxial layer matches the crystal lattice of the substrate at the growth temperature of the epitaxial layer, tensile stress acts on the crystal of the epitaxial layer at the room temperature while if the composition of the epitaxial layer matches the crystal lattice of the substrate at the room temperature, compressive stress acts thereon at the growth temperature. These phenomena occur because the coefficient of thermal expansion of InGaAs is higher than that of InP.
In the thermal diffusion method, not only thermal stress acts on the crystal of the epitaxial layer, but also impurities are diffused in the crystal lattice of the epitaxial layer. As a result, there is a high possibility that the crystal becomes defective and accordingly, there is a high possibility that dark current increases in thermal diffusion process.
The thermal stress is affected by the thickness of the epitaxial layer. That is, the larger the thickness of the epitaxial layer is, the greater thermal stress is. The thickness of the light-receiving portion of the epitaxial layer is related to the sensitivity of the light-receiving elements. Preferably, the thickness of the light-receiving layer is large in terms of sensitivity of the light-receiving elements.